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Bionic JPEG: A libHybris example

(One of the nice things about my new job is working ‘in public’. I’m
tagging posts like these with ‘Canonical’, if you want to filter
them)

Hybris (aka libHybris) is a piece of enabling technology
that lets an OS distribution like Ubuntu use parts of Android software
in binary form, without needing a recompile of those binaries. Ubuntu
is using it for its ARM based Ubuntu Touch distributions, which re-use
the Android BSP for the underlying hardware platform.

libHybris is two things: A dynamic linker, which provides the
generic functionality, and then a set of wrapper libraries that
provide particular Android libraries to the other OS. Whilst the code
is clearly organised, it might be helpful to have a separate sample
which demonstrates how to use the core Hybris features. That is where
Bionic JPEG comes in.

Bionic is the name of Android’s C runtime library. On Ubuntu
the conventional C runtime is glibC. So another way of looking at
Hybris is to regard it as a way to use bionic based binaries in a
glibC based OS. Hence the name of this project.

Bionic JPEG aims to demonstrate how to call a library compiled for use
on Android (In this case the IJG JPEG library) from an Ubuntu
binary. To do this, I’ve divided the IJG code into its library and
client components, then compiled the library with the Android NDK, and
the client with the Ubuntu Touch toolchain. In order for the clients
to use the Android library, a small ‘bridge’ library that calls
libHybris glues the two together. By designing this bridge library to
present the same API as the IJG library, no changes are needed to any
of the IJG source code.

The core of Hybris is clearly derived from the Bionic source, and is a
port of the Android dynamic linker to Ubuntu. This knows how to load
an Android Elf32 binary into an Ubuntu process, which is a trick the
standard Ubuntu dynamic linker can’t do. From the look of the code,
Android can effectively link/prelink binaries in several ways, and it
is loading these binaries that is the key Hybris feature. In addition,
as it resolves symbols present in the binary it loads, it can hook
them to somewhere else. Hybris then uses this to hook all the bionic
entrypoints, and re-direct them to glibC. This isn’t always a simple
symbol substitution – there are differences between the C libraries
that means Hybris has some implementations within it, that then call
on to glibC.

Whilst the IJG code is unmodified, I have changed the name of the
library produced. It turned out that the Android images I was using
(derived from CyanogenMod 10.1 images for the Nexus 4) already have a
copy of libJPEG, from an earlier version of the IJG code. In order to
avoid a collision, Bionic JPEG names its library libjpeg2.

For more details, see the README in the source tarball (available in
the project downloads). Note that I don’t anticipate
Bionic JPEG demonstrates something commonly done by Ubuntu SDK
users. Over time (several release cycles), I think the Ubuntu
community hope to phase out our dependence on Android, in favour of
the common Linux upstreams Ubuntu and Android share. That will take
time, and need us to be successful enough for hardware manufacturers
to offer direct support. For now, using Android gets us that support
for the cost of maintaining Hybris.

That being said, even while libHybris exists, I don’t think it will be
a common thing for people to extend it: for most cases, the Android
libraries on a given device will already have a complete libHybris
bridge.

For the teams that need to maintain those Android parts, there may be
a need to extend libHybris, or even just to understand it a little
better. It is with that use case in mind that I created this example.